Bruce E. Ivins, the chief suspect in the 2001 anthrax mailings, committed suicide before the FBI could present its case in court. Years later, some suspicions remain over results of the inquiry.

Reporting from Washington — FBI Agent Edward Montooth began worrying the moment he got the call early on the morning of July 27, 2008: The chief suspect in the deadly anthrax letter attacks of 2001 had just been rushed to a hospital.

The leader of the FBI investigation knew that if Army microbiologist Bruce E. Ivins died, the opportunity to present the case against him in a courtroom would be lost. Conspiracy theories and speculation, he feared, could well overshadow the evidence.

"They better save [him]," Montooth snapped to a colleague as he hung up the phone.

In previous weeks, Ivins had been warned by his lawyer that he faced an indictment, and the possibility of the death penalty, in connection with the attacks, which killed five people, injured or hospitalized 17 others and helped spur significant changes in national security policies. Ivins died two days after he arrived at the hospital, minutes from his home, in Frederick, Md.

More than three years after Ivins' suicide, Montooth has retired from the FBI, but his earlier concern — that the lack of a trial could fuel suspicions about the government's case — remains valid. Over the last week alone, media reports have questioned anew the evidence against Ivins, while suggesting that the anthrax attacks may have been committed by unidentified wrongdoers.

One account came from three scientists — long critical of the FBI — whose questions were the subject of a story in the New York Times. Another came from the nonprofit group ProPublica, the PBS documentary unit Frontline and McClatchy Newspapers. The coverage highlighted the lingering antagonism toward the FBI among some of Ivins' colleagues at the Army's biowarfare research center at Ft. Detrick, Md.

In response to the reports, FBI spokesman Michael Kortan said the bureau stood by its conclusion that Ivins was the perpetrator, "based both on the scientific findings and the results of the extensive traditional criminal investigation."

Kortan was alluding to the separate branches of the investigation. The scientific evidence against Ivins included tracing the anthrax used in the attacks to a parent flask of spores he created. The conventional investigative efforts found that Ivins had lived a double life: Respected as a scientist but deeply troubled, he had acknowledged homicidal plots to a psychiatrist and a counselor. A year before the letter attacks, Ivins wrote that he might do "terrible things." Other records showed that in the weeks preceding the mailings, he spent unusual late-night hours alone in his specially equipped Army lab.

In the aftermath of Ivins' suicide, the complexity of the scientific evidence has challenged the FBI's ability to satisfy critics. In February, a National Academy of Sciences committee questioned the conclusiveness of some of the evidence. It said genetic tests "were consistent with the finding that the spores in the attack letters were derived from" a batch of anthrax created by Ivins. But, the panel continued, "the analyses did not definitively demonstrate such a relationship."

The documentary produced by Frontline, in collaboration with ProPublica and McClatchy, emphasized the committee's critique of the genetic evidence and suggested that the case might be unsolved. However, Paul S. Keim, a leading geneticist who appeared in the documentary and whose lab tested more than 1,500 anthrax samples for the FBI, said in an interview Thursday that he believed the National Academy of Science's report affirmed the strength of the match to Ivins' flask, labeled RMR-1029.

"The results were consistent with RMR-1029 being the source," said Keim, a professor at Northern Arizona University. If the case had gone to court, he said, "I believe that additional work would have been done to make the linkage stronger."

The renewed attention to the genetic results reflects the role of a still-emerging discipline called microbial forensics. In the anthrax case, it entailed matching the DNA of the spores used in the letters to those from Ivins' batch. The National Academy of Sciences panel faulted the FBI for not using some newly invented techniques to further scrutinize the conclusion of a genetic match. But scientists who were involved in the criminal case noted that only procedures proven to be reliable through multiple experiments could ensure that the resulting evidence could be accepted at a trial.

The three scientists the New York Times wrote about focused on the presence of tin in the anthrax spores sent through the mail. The three theorized that the tin might indicate a method of manipulating anthrax that exceeded Ivins' capabilities. Though their analysis provided no original scientific data, it implied that the mailed spores might have been specially treated to make them easier to inhale and were derived from an undisclosed U.S. biowarfare effort.